SIMPLE ISOLATED CELLS. EPITHELIUM. 



145 



e permanency. It is to be remembered that, in the healthy state of 

 the serous and synovial membranes, and in that of the lining membrane 

 of the blood-vessels and absorbents, they are entirely secluded from 

 sources of irritation ; and that they lead a sort of passive life, very dif- 

 ferent from the active life of the mucous membranes. In fact, it would 

 appear to be the sole object of the serous membranes, to enclose and 

 suspend the viscera, in such a manner as to allow of the access of blood- 

 vessels, nerves, gland-ducts, &c.; and at the same time to permit them 

 the required freedom of motion, and to provide against the irritation of 

 opposing parts, by furnishing an extremely smooth and moistened sur- 

 face, wherever friction takes place. Hence we find membranes, with all 

 the characters of serous surfaces^ in the false joints formed by ununited 

 fractures, and in other similar situations. 



233. The Epithelium of the Mucous membranes and their prolonga- 

 tions, is found under two principal forms, the tessellated, and the cylin- 

 drical. An example of the Tessellated form is shown in Fig. 24, which 

 shows the separate epithelium cells of the mucous membrane of the mouth, 

 as they are frequently met with in saliva. The cells are not always so 

 polygonal in form, however ; sometimes retaining their rounded or oval 

 form, and being separated by considerable interstices, so that they can 

 scarcely be said to form a continuous layer. A specimen of this kind 

 is seen in Fig. 25, which represents a group of epithelium cells from one 

 of the smaller bronchial tubes. This form of tessellated epithelium is 

 more commonly met with, where the secreting operations are more active, 

 the life of the cells consequently shorter, and the renewal of them more 

 frequent ; so that they have not time, so to speak, to be developed into 

 a more continuous layer. The Cylinder-Epithelium is very differently 

 constituted. Its component cells and cylinders, which are arranged side 

 by side ; one extremity of each cylinder resting upon the basement-mem- 



Fig. 25. 



Separated Epithelium-cells, a, 

 with nuclei, b, and nucleoli, c, 

 from mucous membrane of 

 mouth. 



Pavement-Epithelium of the 

 Mucous Membrane of the 

 smaller bronchial tubes; a, 

 nuclei with double nucleoli. 



brane, whilst the other forms part of the free surface. The perfect 

 cylindrical form is only shown, when the surface on which the cylinders 

 rest is flat or nearly so. When it is convex, the lower ends or bases of 

 the cells are of much smaller diameter than the upper or free extremi- 

 ties ; and thus each has the form of a truncated cone, rather than of a 

 cylinder. (Fig. 26). This is well seen in the cells, which cover the villi 

 the intestinal canal. (Fig. 29). On the other hand, where the cylin- 

 Jr-epithelium lies upon a concave surface, the free .extremities of the 

 cells may be smaller than those which are attached. Sometimes each 



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